1. Field of the Invention
The present invention relates to systems and methods for controlling delivery of a pressurized flow of breathable gas to a patient and, more particularly, to a ventilation mask such as a nasal mask, nasal prongs mask or nasal pillows mask for use in critical care ventilation, respiratory insufficiency or PAP (Positive Airway Pressure) therapy and incorporating a piloted exhalation valve inside the mask.
2. Description of the Related Art
As is known in the medical arts, mechanical ventilators comprise medical devices that either perform or supplement breathing for patients. Early ventilators, such as the “iron lung”, created negative pressure around the patient's chest to cause a flow of ambient air through the patient's nose and/or mouth into their lungs. However, the vast majority of contemporary ventilators instead use positive pressure to deliver gas to the patient's lungs via a patient circuit between the ventilator and the patient. The patient circuit typically consists of one or two large bore tubes (e.g., 22 mm ID for adults; 15 mm ID for pediatric) that interface to the ventilator on one end and a patient mask on the other end. Most often, the patient mask is not provided as part of the ventilator system, and a wide variety of patient masks can be used with any ventilator. The interfaces between the ventilator, patient circuit and patient masks are standardized as generic 15 mm/22 mm conical connectors, the size and shape of which are specified by regulatory bodies to assure interoperability.
Current ventilators are designed to support either single limb or dual limb patient circuits. Ventilators using single limb patient circuit are most typically used for less acute clinical requirements, such as treatment of obstructive sleep apnea or respiratory insufficiency. Ventilators using dual limb patient circuits are most typically used for critical care applications.
Single limb patient circuits are used only to carry gas flow from the ventilator to the patient and patient mask, and require a patient mask with vent holes. The pressure/flow characteristics of the vent holes in the mask are maintained according to standards that assure interoperability of masks with a multitude of ventilators that follow the standard. When utilizing single limb circuits, the patient inspires fresh gas from the patient circuit, and expires CO2-enriched gas, which is purged from the system through the vent holes in the mask and partially breathed down the tube to the ventilator and re-breathed during the next breath. This constant purging of flow through vent holes in the mask when using single-limb circuits provides several disadvantages: 1) it requires the ventilator to provide significantly more flow than the patient requires, adding cost/complexity to the ventilator and requiring larger tubing; 2) the constant flow through the vent holes creates noise, which has proven to be a significant detriment to patients with sleep apnea that are trying to sleep with the mask, and also to their sleep partners; 3) the additional flow coming into proximity of the patient's nose and then exiting the system often causes dryness in the patient, which often drives the need for adding humidification to the system; and 4) patient-expired CO2 flows partially out of the vent holes in the mask and partially into the patient circuit tubing, requiring a minimum flow through the tubing at all times in order to flush the CO2. To address the problem of undesirable flow of patient-expired CO2 back into the patient circuit tubing, currently known CPAP systems typically have a minimum-required pressure of 4 cmH2O whenever the patient is wearing the mask, which produces significant discomfort, claustrophobia and/or feeling of suffocation to early CPAP users and leads to a high (approximately 50%) non-compliance rate with CPAP therapy.
When utilizing dual limb circuits, the patient inspires fresh gas from one limb (the “inspiratory limb”) of the patient circuit and expires CO2-enriched gas from the second limb (the “expiratory limb”) of the patient circuit. Both limbs of the dual limb patient circuit are connected together in a “Y” proximal to the patient to allow a single 15 mm or 22 mm conical connection to the patient mask.
In the patient circuits described above, the ventilator pressurizes the gas to be delivered to the patient inside the ventilator to the intended patient pressure, and then delivers that pressure to the patient through the patient circuit. Very small pressure drops develop through the patient circuit, typically around 1 cmH2O, due to gas flow though the small amount of resistance created by the 22 mm or 15 mm ID tubing. Some ventilators compensate for this small pressure either by mathematical algorithms, or by sensing the tubing pressure more proximal to the patient.
Ventilators that utilize a dual limb patient circuit typically include an exhalation valve at the end of the expiratory limb proximal to the ventilator. The ventilator controls the exhalation valve, closes it during inspiration, and opens it during exhalation. Less sophisticated ventilators have binary control of the exhalation valve, in that they can control it to be either open or closed. More sophisticated ventilators are able to control the exhalation valve in an analog fashion, allowing them to control the pressure within the patient circuit by incrementally opening or closing the valve. Valves that support this incremental control are referred to as active exhalation valves. In existing ventilation systems, active exhalation valves are most typically implemented physically within the ventilator, and the remaining few ventilation systems with active exhalation valves locate the active exhalation valve within the patient circuit proximal to the ventilator. Active exhalation valves inside ventilators are typically actuated via an electromagnetic coil in the valve, whereas active exhalation valves in the patient circuit are typically pneumatically piloted from the ventilator.